VARIABILITY IN BEDROCK INCISION RATES IN THE EASTERN COLORADO PLATEAU AND SOUTHERN ROCKIES: EVIDENCE FOR NEOTECTONIC CONTROL ON LANDSCAPE EVOLUTION

The southern Rockies have experienced both dramatic climate change and active tectonism over the last ~3 Ma, but the relative importance of these factors in landscape evolution remains poorly known. River systems are sensitive to these changes and analysis of their profiles and incision history holds promise for separating the effects of tectonics from climate.

River profiles were analyzed to derive bedrock incision rates for key rivers draining the eastern Colorado Plateau and southern Rockies. Incision rates were calculated by using dated river terraces or dated volcanics overlying river gravels. We investigated three systems dominated by separate tectonic provinces 1) Rio Grande, Ojo Caliente, and Chama (rift dominated); 2) Jemez River (Valles Caldera); and 3) Rio San Jose and Puerco (Jemez lineament). Average incision rates vary widely from 30 to 230 m/my over the last 3 m.y. Rates vary through time for individual rivers and between river systems. Some of the variability in bedrock incision rates is interpreted to be caused by tectonics: 1) Incision variability along the Rio Grande corresponds to rift faults, in agreement with evidence that the faults were active over the last 3 m.y.; 2) a three-fold difference in bedrock incision rates between the Jemez and the Ojo/Chama systems between 1.6 and 0.6 Ma (100 vs. 30 m/my) is interpreted to record uplift due to doming in the caldera that triggered rapid bedrock incision on the Jemez, while erupted ash clogged rivers in the Rio Grande valley slowed incision; 3) The highest rates calculated were along the San Jose and Puerco systems (230 m/my), where large volumes of Quaternary basaltic volcanism are interpreted to have produced epeirogenic doming along the Jemez lineament.

River profile analysis identified anomalies that may correspond to incision variability. These anomalies include broad convexities in river profiles, knickpoints, and past floodplain levels with different gradients. Positive trends in the Hack gradient index were identified in the Rio San Jose and Rio Puerco where they cross the lineament, suggesting that gradient increased due to magmatism.

Identification of the cumulative effects of these local tectonic influences on river profiles is building a better understanding of the response of landscape to regional scale neotectonics of the southern Rockies.